feat: capture provider rate limit headers and show in /usage (#6541)

Parse x-ratelimit-* headers from inference API responses (Nous Portal,
OpenRouter, OpenAI-compatible) and display them in the /usage command.

- New agent/rate_limit_tracker.py: parse 12 rate limit headers (RPM/RPH/
  TPM/TPH limits, remaining, reset timers), format as progress bars (CLI)
  or compact one-liner (gateway)
- Hook into streaming path in run_agent.py: stream.response.headers is
  available on the OpenAI SDK Stream object before chunks are consumed
- CLI /usage: appends rate limit section with progress bars + warnings
  when any bucket exceeds 80%
- Gateway /usage: appends compact rate limit summary
- 24 unit tests covering parsing, formatting, edge cases

Headers captured per response:
  x-ratelimit-{limit,remaining,reset}-{requests,tokens}{,-1h}

Example CLI display:
  Nous Rate Limits (captured just now):
    Requests/min [░░░░░░░░░░░░░░░░░░░░]  0.1%  1/800 used  (799 left, resets in 59s)
    Tokens/hr    [░░░░░░░░░░░░░░░░░░░░]  0.0%  49/336.0M   (336.0M left, resets in 52m)

hermes_cli/commands.py

@@ -129,7 +129,7 @@ COMMAND_REGISTRY: list[CommandDef] = [
     CommandDef("commands", "Browse all commands and skills (paginated)", "Info",
                gateway_only=True, args_hint="[page]"),
     CommandDef("help", "Show available commands", "Info"),
-    CommandDef("usage", "Show token usage for the current session", "Info"),
+    CommandDef("usage", "Show token usage and rate limits for the current session", "Info"),
     CommandDef("insights", "Show usage insights and analytics", "Info",
                args_hint="[days]"),
     CommandDef("platforms", "Show gateway/messaging platform status", "Info",